1. Field of the Invention
This invention relates to a photographic camera objective lens system, for example of the Gauss or Tessar type having a tendency to increase various aberrations when the photographic object is relatively close to the lens system, and more particularly to such an objective lens system which incorporates means for compensating aberrations when focussing at a close object.
2. Brief Description of the Prior Art
The prior art is familiar with the aberration problems that are created when it is desired to utilize a lens system for focussing at relatively close objects. For example, a lens system is known wherein an air space behind a diaphragm in the lens system is provided before a positive overall refractive power lens group. This air space is varied in coordination with focussing to compensate for any changes in aberrations. The axial rays passing through this air space are convergent. Thus, by widening this air space the height of the axial rays relative to the optical axis that are incident on the positive lens group can be reduced to accordingly reduce the refraction of the axial rays by the positive lens group and thereby compensate for spherical aberrations. Additionally, the height of the off-axial rays incident on the same positive lens group will also be varied to compensate for astigmatism and coma. An example of such an optical design approach to compensate for these aberrations is disclosed in U.S. Pat. No. 3,815,974.
However, a design problem exists, in that the convergent axial rays in the variable air space become divergent when the object distance becomes sufficiently close. Accordingly, when photographing an object at a relatively close distance a point will be reached in which it is no longer possible to obtain an effective compensation for spherical aberration. If this focussing range is sought to be increased by maintaining the convergence of the axial rays in the variable air space, the lens group at the object side of the variable air space must be provided with a considerably strong positive refractive power. The provision of this power distribution in the lens system, however, would create problems and would not provide a desirable design solution.
Another patent of general interest for its disclosure of various air spaces in a photocopier application can be found in U.S. Pat. No. 3,883,228.
The prior art is still seeking an optical design to optimize the correction of aberrations when focussing at close distances within a commercial priced lens system for the average photographer.